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J Thorac Cardiovasc Surg 2006;132:210-211
© 2006 The American Association for Thoracic Surgery
Letter to the Editor |
Department of Cardiovascular Surgery, University Hospital Zurich, Zurich, Switzerland
Left ventricular assist device (LVAD) support was originally used to bridge high risk patients to heart transplantation.
1
After increased experience with this therapy showed that the failing native heart may have the potential to recover, LVAD therapy provided the option of weaning selected patients from mechanical support as an alternative to transplantation.
2
As technology made further progress, the duration of support has increased markedly and paved the way for destination therapy.
Device selection, however, in particular for long-term support, still remains an unresolved question. Most centers have experience with one or two types of devices. Devices are selected on the basis of personal experience. As long as people are satisfied with the devices they are using, they do not see a reason to change to other systems. Only centers that handle large numbers of patients use a variety of devices, which offers them the opportunity to compare systems. Prospective randomized studies to evaluate different devices for different indications have not yet been performed.
It is of foremost importance that LVADs meet the criteria of long-term reliability and excellent quality of life in addition to the standard requirements of low frequency of infection and thromboembolism. The intracorporeal pulsatile devices such as the Novacor LVAS and the HeartMate I have demonstrated reliability for long-term support.
3,4
Extracorporeal pulsatile devices such as the Thoratec LVAD and the Berlin Heart Excor also have supported patients over the long term.
5,6
The introduction of axial-flow pumps revolutionized mechanical circulatory support. Pump size was markedly reduced, which facilitated the implantation procedure. Bleeding and infectious complications became less frequent. Because axial-flow pumps have not been on the market as long as pulsatile devices, their long-term reliability still has to be proved. Some patients were supported for more than 1 year with the DeBakey VAD, the Jarvik 2000, or the Berlin Heart Incor, respectively.
7-9
In particular, the Berlin Heart Incor promises good long-term performance because of the magnetic suspension of its impeller, which provides virtually unlimited durability. The longest support period with this device is about 3 years. The design of axial-flow pumps renders the exchange of single parts less invasive than is required for larger pulsatile devices.
Axial-flow pumps appear to provide a better quality of life than pulsatile devices. They are much smaller than intracorporeal pulsatile pumps, which makes them easier to implant in the pericardium. There is no need for placement in the abdominal wall or in the abdomen, which may cause gastrointestinal and digestive complications. In addition, the axial-flow pumps run silently, whereas the clatter of pulsatile devices accompanies the patient and his environment day and night. Extracorporeal pulsatile systems are associated with an additional aesthetic and psychological disadvantage. The pump chambers, which are located permanently outside the body, remind the patient of his disease, and people immediately recognize the patient as ill. In addition, personal mobility is limited because of the extracorporeal driving unit. All of these factors limit quality of life.
The greater power of pulsatile devices is often considered an advantage compared with axial-flow pumps. However, experience shows that a flow of 3 to 5 L/min, which is easily provided by axial-flow devices, is sufficient support for daily activities. The continuous flow is well tolerated. This fact may be, at least in part, explained by the flow characteristics. Patients on axial-flow pumps have a less pulsatile flow pattern because the activity of the native heart at rest transmits pulse waves through the pump. This activity is obviously sufficient to reestablish and maintain normal organ function.
Taking all of these factors into account, the authors prefer axial-flow devices for long-term support. For destination therapy, the authors currently use the Berlin Heart Incor, which is being evaluated for this indication in the INDESTINY trial. The management of anticoagulation and platelet inhibition in patients with axial-flow devices still remains an unsolved problem. Experience with the DeBakey VAD, which was the first axial-flow pump on the market, showed that, in such pumps, platelet inhibition in addition to anticoagulation is even more important than in pulsatile devices. In our own experience with the DeBakey VAD, administration of clopidogrel (75 mg) in addition to aspirin (300 mg) and dipyridamol (75 mg) as well as intensification of oral anticoagulation (INR 3.5 to 4.5) reduces the frequency of thromboembolic events.
7
In patients with the Berlin Heart Incor, the regimen was modified. Anticoagulation is titrated at an INR of 2.0 to 3.0, and platelet inhibition consists of aspirin (100 mg) and clopidogrel (75 mg). Anti-platelet therapy is individually guided by different platelet function tests.
We do not yet have the optimal pump for destination therapy. We need pumps with better flow characteristics and fluid dynamics that cause less damage to platelets and reduce the risk of thrombus formation. It will be interesting to see if the implantable centrifugal devices that will soon be on the market have an advantage in this respect. The pumps are required to run reliably and service-free for many years, and their design should allow exchange of worn parts in a minimally invasive intervention. Completely implantable devices with controller and energy supply inside the body, which are currently in development, will further increase quality of life and reduce the risk of infection.
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